The Economist has published an article this week highlighting the work of Lu Wu at the Gwangju Institute of Science and Technology in South Korea in which he and his colleagues have developed a process for producing graphene that could lead to better supercapacitors.
While The Economist article makes some pretty incredible claims, such as that the graphene-based supercapacitors produced by the Korean researchers can store more energy per kilogram than lithium-ion (Li-ion) batteries, the actual research paper in the Journal of Power Sciences offers a less-hyped but nonetheless impressive list of achievements from the research.
The key development is that the researchers developed a non-toxic, low-temperature process for producing graphene that had better electrochemical performance than those synthesized by high-temperature methods. The researchers claim that the energy density of the supercapacitors produced with their graphene achieved a remarkable 131 watt-hours per kilogram (Wh/Kg), nearly four times the previous record for graphene-based supercapacitors.
Of course, using graphene to improve supercapacitors has been the focus of a great deal of research around the world. While much lip service is paid to graphene’s theoretical surface area of around 2600 square meters per gram, which should make it very attractive for storing ions on the surface of a supercapacitor’s electrodes, we rarely hear the other side of the story, namely that this surface area is only possible with a single standalone sheet of graphene.
The problem with this “theoretical surface area” story is that you can’t actually use a standalone sheet of graphene for the electrode of a supercapacitor because it will result in a very low volumetric capacitance. To this day, the best that anyone has been able to achieve for the surface area of an electrode using graphene is around 1500 square meters per gram, which is not much better than the far cheaper activated carbon produced from crushed coconuts.
While scientists continue to try to maximize the potential for graphene in holding a charge on the electrodes of supercapacitors using new twists on the material such as “holey graphene” or graphene/carbon nanotube hybrids, the new direction of research has moved away from a “charge-capacity race” and is instead aiming to exploit graphene’s two real advantages in supercapacitors: its ability to be structured into smaller sizes and its high conductance.
Despite this general research trend, some people still hope that the supercapacitor will provide the answer to powering electric vehicles and solve the vexing problem of lithium-ion (Li-ion) batteries taking hours to charge.
However, the problem with Li-ion batteries is not just slow charging. Back in 2010, then US Energy Secretary Stephen Chu laid out the metrics that would allow Li-ion-battery-powered vehicles to end the supremacy of fossil fuel powered vehicles. Key among these metrics were that Li-ion batteries should have an energy density of 1000Wh/Kg and be three times less expensive to produce.
Today the average Li-ion battery has around 200Wh/Kg—way short of Chu’s goal—but graphene-based supercapacitors are even further off the mark, only achieving around 35Wh/Kg in lab prototypes.
But Wu and his colleagues claim that their graphene had an energy density of 131Wh/Kg. This is certainly a big leap from the 35Wh/Kg that had been previously achieved in supercapacitors, but is still short of a Li-ion’s average energy density of 200Wh/Kg.
Even so, this is a remarkable leap in energy density for supercapacitors and puts storage capacity parity with Li-ion batteries within reach. And when you have a a supercapacitor that has the storage capacity of a Li-ion battery and the ability to charge in mere moments rather than hours, all-electric vehicles might just be a lot more attractive.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.